1. Field of the Invention
The invention relates to non-wood products for use in construction and for use as substitutes for dimensional lumber or corresponding engineered wood products and in the same applications and dimensions as wood lumber products.
The invention also relates to retaining wall systems, and more particularly to reinforced retaining wall systems.
2. Background Prior Art
In the United States, wood lumber products have formed the primary structural elements or building materials for many types of construction, especially in the single- and multi-family housing sector. A large segment of the U.S. home construction industry revolves around the use of common wood lumber framing systems for walls comprising 2×4's or 2×6's placed on 16-inch centers and floors constructed of 2×10's on 16-inch centers. Skilled labor has been trained to assemble these specific types of framing. Special equipment has also been designed and manufactured to perform and speed up the process of assembly. Therefore, any proposed changes in construction techniques that seek to significantly alter established construction practices would not be viewed favorably by the construction industry nor the marketplace. For years, lumber has been abundant and relatively inexpensive. Also, its natural structural properties and its ease of manufacture have assured its dominant position. However, with the growth of the economy, dwindling forest resources, and the emerging significance of global environmental issues such as the greenhouse effect, there is a need to re-assess the widespread use of wood-based products in building construction.
Over the years, many substitute building construction products have been brought into the market with varying degrees of success. However, none of these products are compatible with current methods and techniques for wood frame construction, the large pool of labor skilled in wood-frame construction, and the equipment developed and available to that industry. New concepts have either attempted to change the construction and structural system altogether, or required construction workers to learn new skills and use new forms of equipment to perform the construction work. These prior art concepts also affected conventional ways of handling other aspects of construction such as plumbing and electrical work. For example, replacing the wood frame wall concept with conventional concrete walls or Insulated Concrete Form (ICF) walls requires construction workers skilled in concrete forming, placement, and curing; affects the way the electrical and plumbing work is done; and results in a wall system far heavier than the corresponding wood frame system. Heavier building elements result in higher inertia forces during earthquakes. Walls built with conventional cellular concrete blocks or panels are lighter, but have very low compressive strengths. Because of their brittleness, their response to lateral loading caused by earthquakes in seismic zones or caused by hurricanes or other strong winds is an area of major concern.
Steel studs have been developed and used to approximate wood frame construction. These hollow studs are made of cold-formed steel. They are generally not nailable, although metal screws are used. They are generally not sawable in the field and need to be pre-cut to exact lengths. In contrast to the relative flexibility afforded plumbers or electricians in wood-frame buildings, the steel stud frames have pre-placed positions for the passage of plumbing or electrical hardware. Due to the high thermal conductivity of steel, ghost shadowing, which comprises the appearance of a shadow of the metal stud on the gypsum board wall, has also been a problem. Steel studs can also be susceptible to local or general buckling when subjected to extreme loads or heat.
U.S. Pat. No. 5,479,751 discloses a method and apparatus for fabrication of wood substitute products containing cement and synthetic resin. The disclosed product is described as having sawability and fastener-holding properties. The product includes an outermost casing (hollow tubular body) which is filled with cement and resin. Because the cement mixture inside the tube is not reinforced for tensile stresses, the casing provides that structural function. Because it is common practice to remove parts of the dimensional lumber for fitting and other purposes in wood-frame construction, any cutting of the casing in this product would compromise the structural integrity of the member.
Aerated cellular concrete is a light-weight cement-based product that has been used in some concrete houses. A few commercial manufacturers produce cellular concrete blocks and panels in the United States. However, the structural systems used in such cases are typically based on load-bearing walls, which is a significant departure from framing systems used in wood houses. Cellular concrete is both sawable and nailable. However, special nails are generally recommended to provide nail pull-out capacities. The strength of common cellular concrete is relatively low. Because of its brittleness, fabrication of members such as 2×4's from cellular concrete is not feasible because they would easily break. In general, the ingredients of cellular concrete include Portland cement, silica sand, lime, water, and a foaming agent which is typically aluminum powder. Cellular concrete plants use autoclaves to cure the cast blocks.
The prior art also includes fiber-reinforced concrete, and significant research has been performed particularly in the last decade on various applications of fiber-reinforced concrete including the use of fiber-reinforced cellular concrete building panels for construction of an envelope surrounding buildings for protection against hurricane-induced missiles. Fiber reinforced cellular concrete has included polypropylene fibers added to cellular concrete to produce 4-in. thick panels. Although this material exhibits improved toughness and ductility which are good properties against missile impact, its compressive strength is low (250 psi or approximately {fraction (1/20)}th of conventional concrete).
U.S. Pat. No. 5,002,620 discloses a laminated or sandwiched panel system in which layers of fiber-reinforced concrete are cast against each other. The layers include a dense layer without air bubbles sandwiched with a lighter layer of cellular concrete. A vapor barrier is placed between the two mating layers. The dense layer of non-cellular material serves as the structural, load carrying element while the cellular layer provides insulation qualities. The fiber-reinforced cellular material discussed in U.S. Pat. No. 5,002,620 does not provide the necessary structural strength to permit use of this product in the form of dimensional lumber and as a primary structural element.
It is important to realize that in wood-frame construction, the imposed loads are being carried by the relatively small cross-sectional areas of the 2×4's or 2×6's as opposed to a wall system where a relatively large area and moment of inertia supports the load. Stress levels are far higher in dimensional lumber members than in a wall system. This substantially increases the strength requirements for the dimensional lumber member. The increased strength must be accommodated in the design of the lumber member. In addition to the strength issue, the nailability, sawability, and weight issues are other restricting factors in a dimensional lumber member. For example, the likely result of attempts to increase compressive strength would be a reduction in nailability and sawability, and an increase in weight. Attempts to increase tensile strength through addition of more fibers leads to dispersion problems and other issues that must be resolved.
U.S. Pat. No. 4,351,670 and U.S. Pat. No. 4,465,719 disclose methods of making, and structural elements incorporating, a lightweight concrete. The lightweight aggregates for this concrete consist of broken-up pieces of cellular concrete that are coated with cement slurry. This material does not include fibers, and can be cast in a casing to form a composite building element. This invention is intended to introduce a new source of lightweight aggregate for concrete.
U.S. Pat. No. 5,685,124 discloses a folded plate panel using boards made of wood. Veneers are attached to one or both sides of the ridges of the folded plate. The hollow spaces thus created are filled with sound- and heat-insulating materials. Lightweight concrete and foamed concrete can be used as insulation filling the hollow spaces. The concrete is not intended to serve a structural function in this invention.
U.S. Pat. No. 2,156,311 discloses a “cement-fibrous” lightweight material with fireproof and waterproof properties based on wood pulp and cement. The patent describes a manufacturing process involving filtering to remove water and roller forming of cement panels. This material is not an aerated cellular concrete.
U.S. Pat. No. 2,153,837 discloses the addition of a small amount of wood pulp to achieve uniformity in cellular concrete walls. The wood pulp is not intended to serve a structural function, but to ensure uniformity of the final product.
Segmented retaining wall systems generally consist of heavy weight concrete or stone blocks placed in layers such that each layer is set back a small distance with respect to the layer below. These systems are referred to as “gravity walls” and typically include blocks that have an interlock device such as a flange or projection on the bottom face of a block that locks with a groove, slot, or mating surface on the top face of a lower stacked block.
Stability of the retaining wall is dependent on the mass of the wall and the amount of setback between stacked blocks. The weight of the backfill behind the retaining wall creates a moment to overturn the retaining wall, a force to slide the base out relative to the ground, and a force to slide each individual layer of main blocks out relative to an adjacent block. The overturning moment is resisted by the weight of the wall, and the sliding forces are resisted by friction between the underside of the base block and the soil and the friction and the interlocking device between adjacent layers of the blocks.
Cast-in-place reinforced concrete cantilever wall systems typically include internal steel bars that provide the necessary strength along the height of the wall. The cast-in-place wall systems generally include a continuous reinforced concrete footing under the wall to distribute the overturning moment and sliding forces to the surrounding backfill. The stability of the wall is dependent on the overall weight of the wall and the weight of the portion of the backfill that is resting directly on top of the footing.
These conventional concrete retaining walls are susceptible to cracking due to poor freeze-thaw durability. The concrete blocks used in the conventional retaining walls are difficult to handle and transport because they are generally heavy and brittle resulting in increased handling costs. Specifically, these blocks typically weigh approximately 150 pounds per cubic foot and will likely shatter when dropped from a relatively small distance onto a hard surface.